Bedrock Geochemistry and Alteration History of the Clay‐Bearing Glen Torridon Region of Gale Crater, Mars

Glen Torridon is a topographic trough located on the slope of Aeolis Mons, Gale crater, Mars. It corresponds to what was previously referred to as the “clay‐bearing unit,” due to the relatively strong spectral signatures of clay minerals (mainly ferric smectites) detected from orbit. Starting in January 2019, the Curiosity rover explored Glen Torridon for more than 700 sols (Martian days). The objectives of this campaign included acquiring a detailed understanding of the geologic context in which the clay minerals were formed, and determining the intensity of aqueous alteration experienced by the sediments. Here, we present the major‐element geochemistry of the bedrock as analyzed by the ChemCam instrument. Our results reveal that the two main types of bedrock exposures identified in the lower part of Glen Torridon are associated with distinct chemical compositions (K‐rich and Mg‐rich), for which we are able to propose mineralogical interpretations. Moreover, the topmost stratigraphic member exposed in the region displays a stronger diagenetic overprint, especially at two locations close to the unconformable contact with the overlying Stimson formation, where the bedrock composition significantly deviates from the rest of Glen Torridon. Overall, the values of the Chemical Index of Alteration determined with ChemCam are elevated by Martian standards, suggesting the formation of clay minerals through open‐system weathering. However, there is no indication that the alteration was stronger than in some terrains previously visited by Curiosity, which in turn implies that the enhanced orbital signatures are mostly controlled by non‐compositional factors. Plain Language Summary In early 2019, the Curiosity rover embarked on a multi‐year exploration of the Glen Torridon region, a topographic trough on the flank of Aeolis Mons, the sedimentary mound inside Gale crater, Mars. Orbital instruments have detected clay minerals over Glen Torridon, making it a primary target for Curiosity, because clay minerals are indicators of past interactions with water, and provide a medium favorable to the preservation of organic compounds. Understanding the context of formation of the clay minerals, and determining how much the sediments have been altered by water were among the objectives of the Glen Torridon campaign. Based on the chemical compositions measured by the ChemCam instrument, we show that the amount of water that interacted with the sediments was high enough to take a